1. Field of the Invention
The present invention relates to a polyimide resin known as a heat-resistant resin and, more particularly, to a novel copolyimide possessing excellent thermal dimensional stability and mechanical properties.
2. Description of the Prior Art
Polyimides are well known as polymers having excellent heat resistance. These polymers also have good resistance to chemicals, as well as having good electrical and mechanical characteristics. A typical polyimide is a polymer prepared by means of condensation polymerization, using 4,4'-diaminodiphenyl ether and pyromellitic dianhydride, and which is commercially available in quantity. This polymer is used as an electrical material requiring heat resistance, such as a flexible printed circuit board. Although the polymer has good mechanical characteristics such as high tensile properties, its thermal dimensional stability is poor (linear thermal expansion coefficient: 3.times.10.sup.-5 .degree. C..sup.-1), which leads to problems such as warping and curling of flexible printed circuit boards. A flexible printed circuit board comprises a polyimide film and a metal laminated thereon. Since the linear thermal expansion coefficient of the metal is less than that of the polyimide film, warping and curling occur due to changes in temperature during the fabrication and subsequent use of the flexible printed circuit boards.
Another problem caused by poor thermal dimensional stability of polyimide is warping or curling of a magnetic recording material. A recent high-density magnetic recording material is fabricated by depositing a metal on a base film. Since metal deposition is performed at a high temperature, a heat-resistant polymer such as a polyimide should preferably be used as the base film. However, since the linear thermal expansion coefficient of the polyimide film is greater than that of the metal, undesirable warping and curling inevitably occur.
Since polyimides have good heat resistance, they receive a great thermal influence. Consequently, demand has arisen for the fabrication of a polyimide which possesses excellent thermal dimensional stability. With the advent of electronics, in particular, demand for such has only become stronger.
Examples of a polyimide having excellent thermal dimensional stability are disclosed in Japanese Patent Disclosure (Kokai) Nos. 61-158025, 61-181828, 61-241335, and 61-264028. In the polyimides disclosed in Japanese Patent Disclosure (Kokai) Nos. 61-158025 and 61-264028, biphenyltetracarboxylic dianhydride and pyromellitic dianhydride (the latter is used if necessary) are used as tetracarboxylic dianhydrides, and paraphenylene diamine and 4,4'-diaminodiphenyl ether (the latter is used if necessary) are used as diamines. These materials are polycondensed to obtain the above polyimides. The polyimide disclosed in Japanese Patent Disclosure (Kokai) No. 61-181828 is prepared by using a specific heterocyclic diamine (e.g., 2,5-diaminopyridine) as diamine and polycondensing it with an aromatic tetracarboxylic dianhydride. The polyimide disclosed in Japanese Patent Disclosure (Kokai) No. 61-241325 is prepared by using 9,9-bis(4-aminophenyl)anthracene and paraphenylene diamine as diamines and polycondensing them with biphenyltetracarboxylic dianhydride as a tetracarboxylic dianhydride. These polyimides are prepared by using expensive compound materials such as biphenyltetracarboxylic dianhydride, heterocyclic diamine, or 9,9-bis(4-aminophenyl)anthracene.